Electrical power generators convert mechanical power into one or more electrical currents, one from each coil, or winding. It is beneficial to protect electrical power generators against external faults and internal faults. Generators can be protected against external faults by several circuit breakers that isolate the faults that could occur in the power plant network (e.g., transformers, buses, lines . . . ). At the same time, electrical power generators can also be protected against faults that could occur inside the machine itself.
Many electrical power generators in power plants are of a three-phase design that utilizes a “Y” winding configuration. In this type of design, all three phases of the stator are connected at one end, typically by a large bus bar, which corresponds to the center of the “Y”. Usually, this bus bar is also grounded. In most instances the grounding point that is selected is the power plant's earth ground and the bus bar is connected to the ground through a transformer that is commonly referred to as a neutral grounding transformer.
In an ideal operation, the current flow to ground would hypothetically be nearly zero. A very small current will flow to ground through the generator side of the neutral grounding transformer, largely due to capacitance of the complete generator, and all components tied to it electrically. When a fault to ground occurs in any of these electrically connected components the current will rise and can reach levels up to 20 Amperes. The transformer is typically designed to limit ground fault current to 20 Amperes to minimize consequential damage from the ground fault.
For example, a ground fault in a generator stator winding is one of the most frequent types of internal electrical generator fault. A ground fault can be caused by, for example, physical damage to the stator winding or aging of the insulation of the stator and can cause additional damage to the stator and cause the electrical power generator to fail.
In the past, the current passing through a neutral grounding circuit (e.g., that includes a neutral grounding transformer and an impedance) has been measured by monitoring the voltage across that impedance. When the voltage exceeds a predetermined threshold value a protection relay will trip after a short time delay. The short time delay is typically selected to allow spurious conditions to clear in order to prevent false trips of the protection relay which would unnecessarily stop operation of the electrical power generator.
The protection technique of using a relay relies simply on a detected voltage level and may miss other events that are evidence of a fault or are evidence of an impending fault. There remains a need for additional techniques and systems to more accurately and more robustly monitor an electrical power generator in order to provide fault protection other than a simple over-voltage trip relay.